1. Field of the Invention
This invention relates to a blocking laminated film obtained through an inflation film molding process using a thermoplastic resin and a manufacturing method for the same.
2. Description of the Prior Art
In general, packaging materials for high technology products such as photographic photosensitive materials are required to have various properties including gas barrier, moistureproofness, physical strength such as breaking strength, tear strength, impact puncture strength, Gelbo test strength and wear resistance, heat sealing properties such as heat seal strength, hot tack properties (hot-seal ability) and seal ability with other materials, antistatic property, flatness, and the like. It is difficult to satisfy these properties with a single film material, and therefore, the packaging material is usually composed of two or more layers.
In order to improve the physical strength of packaging materials, the inventor developed a cross laminated film comprising two uniaxially stretched thermoplastic resin films laminated directly or through an adhesive layer so that their molecular orientation axes cross each other (U.S. Pat. No. 4,331,725). Another cross laminated film which has been developed contains a foamed sheet interposed between the above uniaxially stretched films (U.S. Pat. No. 4,565,733). In addition, the inventor has investigated in order to further improve the quality and cost of packaging materials, and has developed a packaging material of a multilayer laminated film excellent in physical strength and inexpensive where the inner layer of a coextruded multilayer inflation film is joined utilizing a blocking phenomenon (which is considered to be a serious problem in a inflation process) by using a special resin composition as the inner layer of the inflation film (U.S. Ser. No. 07/218,908).
However, in the conventional multilayer laminated film where a coextruded T die film or inflation film was laminated through an adhesive layer, there are many problems, such as great trimming loss, low flexibility, low bag rupture strength, low tear strength, great curling and the like. The cross laminated films are excellent in bag rupture strength. However, it has also various problems, such as being expensive and having inferior heat sealing properties. Moreover, when respective layers are completely adhered by adhesive layer, tear strength and the like decrease, and curling greatly occurs.
On the other hand, in the case of the laminated film made of an inflation film of which the inner layer is joined by blocking, no adhesive layer nor lamination process is necessary. Moreover, since the above laminated film can be made without cutting both sides different from conventional inflation films, edge trim loss is not produced. The laminated film cost per unit area is sharply decreased, and nevertheless, physical strength is greatly improved. However, when a flexible sheet is laminated to this blocking laminated film, unless the area joining the inner layers of the inflation film to each other by blocking is rendered more than 95%, wrinkling and the separation at the blocking portion frequently occur in the lamination process caused by the air remaining in the unjoined portion. The adhesive strength obtained by the blocking is liable to vary by the cooling conditions and the room temperature and humidity during molding by the inflation process, and the resin composition of the inner layer must be limited strictly. Moreover, in the case that the resin composition of the outer layer is similar to the inner layer, blocking occurs not only between the inner layers but also between the outer layers facing to each other when the inflation film is wound after cooling. Such a laminated film cannot be used. While, when the outer layer is controlled so that blocking does not occur, it is difficult to join the inner layers by blocking.